A wide variety of substances have been proposed for use as fat substitutes in food compositions. The chemical structures of such substances are selected such that they are more resistant to breakdown by the metabolic processes of the human digestive system which normally occur upon ingestion of conventional triglyceride lipids. Because of their increased resistance to digestion and absorption, the number of calories per gram available from the fat substitutes is considerably reduced as compared to common vegetable oils, animal fats, and other lipids. The use of such substances thus enables the preparation of reduced calorie food compositions useful in the control of body weight.
U.S. Pat. No. 4,861,613 describes one class of particularly useful fat substitutes wherein-a polyol such as glycerin is alkoxylated with an epoxide such as propylene oxide and then esterified with any of a number of fatty acids or fatty acid equivalents to form an esterified alkoxylated polyol. Generally speaking, it is desirable to accomplish nearly complete esterification (i.e., to react at least 90% of the hydroxyl groups of the alkoxylated polyol intermediate with fatty acid). These substances have the physical and organoleptic properties of conventional triglyceride lipids, yet are significantly lower in available (absorbed) calories than edible oils owing to their pronounced resistance towards pancreatic lipase enzymatic hydrolysis. The thermal and oxidative stability of the esterified alkoxylated polyols renders them especially suitable for use in the preparation of reduced calorie food compositions requiring exposure to high temperatures.
The methods developed to date for the preparation of esterified alkoxylated polyol fat substitutes of this type have largely required multi-step procedures when a naturally occurring triglyceride is to be utilized as the source of the long chain acyl groups incorporated into the esterified alkoxylated polyol. The triglyceride is first hydrolytically split into glycerin (which may be employed as the polyol component) and a mixture of fatty acids. The fatty acids (after separation from the glycerin) may be used directly without further modification as described in U.S. Pat. No. 4,983,329. Alternatively, the fatty acids prior to use in an esterification reaction with an alkoxylated polyol may be converted into C.sub.1 -C.sub.4 alkyl esters (as described in U.S. Pat. Ser. No. 5,175,232) or fatty acid halides (as described in U.S. Pat. No. 4,861,613). The alkoxylated polyol must first be prepared by reacting an epoxide with a polyol such as glycerin, sugar alcohol, glycoside, monosaccharide, disaccharide or other organic compound having two or more hydroxy groups. While such multi-step procedures work well and afford esterified alkoxylated polyols suitable for use as fat substitutes, the number of steps involved, including both synthetic and purification steps, renders these substances considerably more costly than the triglycerides on which they are based. Since the esterified alkoxylated polyol is intended to entirely or substantially replace conventional high caloric triglycerides in food compositions and since certain types of food compositions will normally contain high levels of fat or oil, it is apparent there exists a great need for improved processes whereby the manufacturing cost of esterified alkoxylated polyols may be substantially reduced.
Although the direct esterification method as described hereinabove and in U.S. Pat. No. 4,983,329 has the advantage of utilizing free fatty acids, thus avoiding the necessity of first preparing alkyl ester or halide derivatives prior to the alkoxylated polyol esterification step, it is not an ideal method. In particular, direct esterification normally is optimally carried out under conditions such that the water generated by the reaction of fatty acid and alkoxylated polyol is continuously removed from the reaction mixture by means such as distillation (which may be under vacuum) or sparging with an inert gas such as nitrogen. Substantially complete esterification is difficult to achieve unless the water is so removed since esterification is an equilibrium reaction. Under typical reaction conditions, the free fatty acids which are present will have a tendency to steam distill together with the water and consequently be removed from the reaction zone. This adversely affects the rate of the desired esterification reaction, not only because the fatty acid is a required reactant but also because such esterifications are preferably conducted in the absence of any metallic catalysts (due to difficulties in removing such catalysts, many of which are highly toxic, after completion of esterification) using an excess of fatty acid to self-catalyze the esterification. If fatty acid is being continually lost from the reaction zone due to overhead losses, fresh fatty acid must be added to maintain the desired rate of reaction. The need to use make-up quantities of fatty acid and to recycle, recover, or otherwise dispose of the fatty acid lost overhead increases the overall cost of such a process. Another problem is that the presence of large quantities of free fatty acid during the entire time necessary to convert an alkoxylated polyol containing no acyl groups initially to a completely esterified product provides an opportunity for the fatty acids to dimerize, polymerize, oxidize, or otherwise degrade. These undesired side-reactions of the fatty acid may result in the formation of acidic or unpleasant-tasting or smelling impurities which are difficult to remove by standard edible oil refining techniques such as deodorization, hydrogenation, or bleaching. The development of synthetic procedures which would alleviate the aforesaid difficulties would therefore be a highly desirable improvement in the fat substitute art.